1. Field of the Invention
This invention relates to underdrains and gravity filters and, more particularly, to a filter media support system for underdrain blocks.
2. Description of Related Art
Granular media filtration units are typically used to filter water, wastewater and industrial fluids. Filtration units typically employ underdrain systems to collect filtrate, channel it away from the filter bed, and also to distribute backwash gases and fluids into the filter bed. Several underdrain assemblies for filter bottoms are known in the art. U.S. Pat. No. 4,619,765 to Roberts; U.S. Pat. No. 5,108,627 to Berkible; U.S. Pat. No. 5,328,608 to Bergmann and U.S. Pat. No. 5,489,388 to Brown describe known underdrain systems. A typical filter media bottom comprises a main flume with multiple parallel rows of filter blocks, also known as laterals, arranged perpendicularly to the main flume across the filter bottom. Filter media of varying particle size covers the filter bottom. Frequently a layer of gravel separates the filter media and the filter blocks to prevent the filter media from penetrating apertures in the top of the filter blocks.
Fluid to be filtered typically enters the filtration unit from above and flows down through the filter media by gravitational forces. The filtrate then flows into the block underdrains, through the main flume to the outlet. The filter media is typically cleansed at predetermined intervals by backwashing. During the backwash cycle, gas and/or liquid is directed through the main flume to the lateral underdrain blocks and upwardly through the filter media to loosen and remove contaminants collected by the filter media. The gravel support layer above the filter blocks may typically be up to 18 inches in height and contain several layers of varying size gravel. The gravel may be layered according to size in an hour glass configuration in which a fine size gravel layer is sandwiched between layers of progressively larger gravel sizes. The innermost fine size gravel layer prevents the filter media from penetrating the underdrain block, while the coarse size layer prevents plugging of the fine gravel layer. Gravel, however, may be difficult and costly to install and may require the use of deeper filter units. In addition, the hour-glass configuration of the gravel layers may be disturbed during backwash, necessitating restoration of the desired layered configuration. In other prior art arrangements, filter media retaining caps supported on the filter blocks may be used in conjunction with, or as a substitution for, the gravel support layer to block the media from entering the filter blocks. Several filter media retaining caps are known in the art.
U.S. Pat. No. 2,716,490 to Barstow discloses a means for securing porous plates to the supporting filter block so that they may be quickly and easily set and held in position to provide an integral porous partition between upper and lower chambers.
U.S. Pat. No. 4,882,053 to Ferri discloses a porous filter support plate of the kind used in traveling bridge filters for the support of granular filter media. The support plates are formed of porous, heat-fusible materials, for example, a thermoplastic organic material, joined together by heat fused butt joints and/or reinforced by vertical zones which extend vertically through the plates in which the material has been brought to a molten state and pressed together to form a dense, solid, nonporous mass.
U.S. Pat. No. 5,019,259 to Hambley discloses a filter underdrain apparatus comprising plate means and a screen arrangement which may be included to screen the liquid and gas orifices from filter media exterior of the distributor conduits. The screen may comprise perforated grids and may extend across the trough between adjacent distributor conduits.
U.S. Pat. No. 5,089,147 to Ross discloses a bed of granular medium such as sand supported on a screen within a filter tank cell. An underdrain structure supports the screen while a hold down grating secures the screen in place from above. The grating is held in place by adjustable hold down means secured to tank walls.
U.S. Pat. No. 5,149,427 to Brown discloses a cap for filter underdrain blocks, wherein the cap has a porous body. The cap is installed on a filter block having a plurality of orifices in a top wall of the filter block for channeling fluids to and from an overlying filter media. The cap eliminates the need for a separate gravel support layer to be installed between the fine grain filter media and the underdrain blocks.
U.S. Pat. No. 5,269,920 to Brown discloses a cap for underdrains and gravity filters which has a top surface and a bottom surface with a plurality of tapered screen members defining slots in the top surface. The slots provide communication with a filter bed without passage of the fine grain filter media therethrough.
International Application WO 97/40907 discloses a system for supporting fluid-treatment media above a lower surface that reduces media clogging and head loss in granular fluid-treatment media systems by providing a layered porous plate. The porous plate can have multiple layers of fine sized and course sized pores. The system for supporting fluid-treatment media is securely anchored to the infrastructure of the underdrain.
It is an object of the present invention to provide a cap for underdrain blocks which support fine grain filter media without resulting in an excessive pressure drop.